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Any project manager who has prepared for the PMP examination or attended a project-scheduling course is familiar with the Program Evaluation Review Technique formula, better known as PERT. Putting aside the theory, network diagrams and standard deviations, here is a tactical application of PERT in schedule development that can help overcome “estimation hesitation.” The Program Evaluation Review Technique (PERT) formula isn’t just a theoretical equation to estimate effort or a fact to memorize for the PMP exam. PERT has practical applications in effort estimation and can be a useful technique when historical work estimates are unavailable or team members are reluctant to commit to an estimate.
When building a project schedule, the task effort estimation is fairly simple for routine tasks with well-defined outcomes. Past project metrics, estimation templates and industry averages can be gathered to estimate common tasks. However, the PERT technique is most useful when standard estimates are unavailable or when project team members are reluctant to provide an estimate.
On several projects, I’ve worked with project team members who were reluctant to provide an estimate despite assurances that no one will be penalized for missing the estimate. It is a common to hear “tell me when you need it done and I’ll deliver it by that date.” This approach either over extends the project schedule or requires the project manager to be the sole estimator.
The PERT technique is an effective approach to overcome estimation hesitation and provide an opportunity to incorporate the best estimate, the worst estimate, and the most likely estimate under common work conditions. The expected time estimate using the PERT formula is defined as:
Expected Time = (Optimistic + 4 * Most Likely + Pessimistic) / 6
For a given task, team members can provide the most aggressive or most optimistic estimate under ideal conditions, the average or most likely estimate, and the most conservative or pessimistic estimate. The PERT formula simply provides a weighted average of the best, worst and most likely estimates.
Some project managers are reluctant to adopt PERT and assume they need to collect the three parameters and apply the formula to every task. In tactical effort estimation, applying PERT to the tasks that are difficult to estimate instead of the entire plan is a useful technique. PERT isn’t necessary to estimate the duration for a project status meeting or the time required to create a project repository on the company file server. These are routine tasks with easy to estimate durations.
The PERT approach is also criticized for its subjective nature and dependency upon judgment instead of metric-driven estimation standards. Depending on the team of “experts” providing the estimates, the estimates may be biased. In any project plan, alternative paths can become the critical path depending on the schedule performance and the PERT estimate becomes underestimated. However, these issues are managed by the project control processes and an examination of schedule variances on a weekly basis. Estimates are simply educated guesses with planned start and end dates. The project schedule still needs to be monitored accordingly.
PERT and Microsoft Project Fortunately, project managers don’t need to run the PERT equation for every task in the plan. Microsoft Project 2003 provides a simple approach to overcome estimation hesitation by following these simple steps:
- Add the PERT Analysis tool bar, by clicking on View – Toolbars – Pert Analysis.
- The PERT Analysis Tool bar is displayed (Fig. 1).
- Click PERT Entry Sheet.
- For each task requiring a PERT estimate, enter the optimistic, expected, and pessimistic durations in the respective fields.
- Click Calculate PERT to calculate the estimated duration for each task.
- The estimated PERT value will be copied into the Duration field.
 Fig. 1: PERT Analysis Tool Bar
Alternatively, tasks can be entered in the default Gantt Entry view and when a PERT estimate is required, the project manager can calculate it by clicking on the PERT Entry Form, entering the estimates, and clicking the Calculate PERT button. The Pert Analysis tool bar can display the Gantt charts based on optimistic, pessimistic and expected estimates. The PERT weights can also be adjusted from the standard formula by clicking on the PERT Weights icon.
This article was written by Andy Makar and was originally published on http://www.projectsatwork.com/content/articles/229535.cfm
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